The detection of newborn babies with potentially life-threatening, critical congenital heart defects (CCHDs) before they collapse or expire remains an important clinical challenge. The absence of physical signs and the difficulty assessing mild cyanosis means that the newborn baby check misses up to a third of babies. Fetal anomaly ultrasound scanning identifies an increasing proportion, but this screen is operator-dependent and therefore highly variable; although some units report very high detection rates, overall most babies with CCHD are still missed. Pulse oximetry screening (POS) is an additional test that meets the criteria for universal screening. POS increases overall detection of CCHD to over 90% and also identifies babies with noncardiac, hypoxemic conditions (such as congenital pneumonia, early-onset sepsis, and pulmonary hypertension), which are usually included in the false positives. There is a wealth of published data on the POS, both in a research setting and more recently in routine clinical practice, and consideration of POS is becoming increasingly widespread particularly among high-income countries. But a degree of controversy still remains, and debate continues regarding the most appropriate time to screen, the most effective screening pathway, and screening outside the well-baby nursery. So, should all newborn babies be screened with POS, if so, when and where should screening take place, what saturations are acceptable, and which conditions are we trying to identify? This review will look at the available evidence and try to suggest the way forward for those considering its introduction into their clinical practice.